Many food products, such as fruit juice, pasteurized or UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated band packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper, or mineral-filled polypropylene material; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging units, on which a continuous tube is formed from the web-fed packaging material; the web of packaging material is sterilized in the packaging unit, e.g. by applying a chemical sterilizing agent such as a hydrogen peroxide solution, which is subsequently removed, e.g. by heating and evaporation, from the surfaces of the packaging material.
The sterilized web is maintained in a closed, sterile environment, and is folded into a cylinder. In particular, the opposite lateral edges of the cylinder are overlapped and sealed to form a longitudinal sealing band of the tube.
The tube is fed in a vertical direction parallel to its axis, and is filled continuously with the sterilized or sterile-processed food product.
Packaging units are known which interact with the tube to heat seal it at equally spaced cross sections to form a series of pillow packages connected to one another and by transverse sealing bands, i.e. extending in a direction perpendicular to the feed direction of the tube.
More specifically, the unit comprises two forming assemblies movable along respective runners and which interact cyclically and successively with the tube to heat seal the packaging material of the tube.
Each forming assembly comprises a slide movable back and forth along the respective runner; and two jaws hinged at the bottom to the respective slide, and movable between a seal position cooperating with the tube to heat seal it, and a rest position detached from the tube.
The movements of the forming assemblies are offset by a half-period: the slide of one forming assembly moves upwards with its jaws in the rest position, as the slide of the other forming assembly moves downwards with its jaws in the seal position to prevent collision and interference.
The jaws of each forming assembly have respective sealing members which cooperate with opposite sides of the tube and comprise, for example, a heating member, and an elastomeric member for providing mechanical support to grip the tube to the required pressure.
Each forming assembly also comprises two forming members comprising respective forming half-shells hinged to the respective jaws.
Each pair of forming half-shells moves cyclically between an open position detached from the tube, and a closed position contacting the tube to fold it between two consecutive sealing sections and so define and control the volume of the package being formed.
More specifically, the sealing members of a first forming assembly seal the package at a bottom sealing band thereof, and the half-shells of the first forming assembly control the volume of the package being formed, and the sealing members of the second forming assembly seal the package at top sealing band thereof.
Known packaging unit also comprises, as shown for example in EP-A-1445196, for each forming assembly, a pair of folding flaps movably fitted to one of the jaw of the corresponding forming assembly and adapted to form triangular flaps at the bottom corners of the pillow packages.
In particular, when the jaws of each forming assembly are in the respective seal position, the relative folding flaps move between:                an open position, in which they are detached from the tube; and        a closed position, in which they cooperate with the tube, so as to form the triangular flaps at the bottom corners of the pillow packages.        
In other words, the folding flaps move for the same angle both:                from the open position to the closed position before that the sealing band is formed; and        from the closed position to the open position, once that the sealing band is formed so as to release the formed package.        
Though highly reliable and efficient, the above-described packaging unit leave room for further improvement.
As a matter of fact, increasing use is made of ultrasonic sealing devices, which substantially comprise a mechanical-vibration generator or sonotrode and an anvil—known for example from EP-B-615907 in the name of the same Applicant—fitted to respective jaws of each pair and which cooperate with each other to heat the packaging material by means of ultrasonic vibrations.
Ultrasonic sealing device are especially used when the packaging unit is required to form packages with no layer of aluminium or other electrically conductive materials.
When the ultrasonic sealing device is used, the sealing members of the first forming assembly move further downward, with respect to the sealing members of the second sealing assembly, after that the transversal sealing band is formed, in order to avoid interference.
This further downward movement may induce stretching on the formed package, which has its top sealing band still retained by the sealing members of the second sealing member, and its bottom sealing band and triangular flaps potentially interfering with the folding flaps of the sealing member of the first folding assembly, especially when the packaging material is stiff.
As a result of this tendency and of the prolonged stretching action, there is thus the risk that the folding flaps in the open position could collide with the triangular flaps of the package in formation, thus causing the package to swing or to be damaged.
The Applicant has recognized that, in order to avoid this collision, the optimal solution would be moving the folding flaps for a higher first angle between the closed position and the open position, thus mitigating the risk of interference with the transversal sealing band and the triangular flaps.
However, this higher first angle between the closed position and the open position could generate some drawbacks, when the folding flaps move from the open position to the closed position and when the sealing members close on the tube to form the transversal sealing bands.
As a matter of fact, the Applicant has also recognized that a smaller second angle between the open position and closed position of the folding flaps would be optimal to contain the risk that the tube of packaging material twists about it own axis when the sealing members close on the tube to form the transversal sealing bands.
This twisting could render unstable the overlapping of the edges of tube intended to form the longitudinal sealing band, during the formation of this longitudinal sealing band of the tube.
Furthermore, a smaller second angle between the open position and the closed position of the folding flaps would be optimal for holding the tube during the formation of the package, thus avoiding the risk of cracking the areas of the packaging material close to the transversal sealing bands and/or the bending of opposite lateral edges of the transversal sealing bands.
A need is felt within the industry to obtain a packaging unit which is particularly flexible as regards to the movement of the folding flaps between the relative open positions and closed positions, so as to properly form, for example, packages from particularly stiff packaging material and/or by using ultrasonic sealing device for forming the transverse sealing bands.
It is an object of the present invention to provide a packaging unit for producing sealed packages from a tube of a sheet packaging material, designed to meet the above requirements in a straightforward, low-cost manner.